The present invention generally relates to a direction-finding system and more particularly to a direction-finding system which uses a plurality of hydrophones in a crossed dipole configuration for locating acoustic targets generating high acoustic frequencies.
The directivity pattern of a monopole such as a hydrophone is an omnidirectional pattern whereas the directivity pattern response of a dipole pair which is a pressure-gradient system, has a figure-8 pattern. The maximum sensitivity in pressure-gradient systems occurs at the frequency where the ratio of the spatial separation L between the members of the dipole and the wave length of the acoustic signal, i.e., L/.lambda., is equal to 1/2. However, the sensitivity of such a system decreases at the rate of 6dB/octave as the frequency decreases. Consequently, a given spatial separation L between the two hydrophones of the dipole may be too small for low frequency work. At the same time this given separation L may be usually too large for high frequency work. For, somewhat above the dipole peaking frequency where L is equal to .lambda./2, grating lobes appear in addition to the simple Cos .theta. and Sin .theta. patterns, where .theta. is the bearing angle. That is, the directional response shows a multi-lobed structure when L is greater than .lambda./2. Hence, ideally L should be inversely proportional to the signal frequency, being very large at the low frequencies and very small at the high frequencies.
For the high frequency problem, if L cannot be made small, it would be desirable to find a way to use a relatively large value of L and still have no grating lobes. This means that d or L/2 (half the spatial separation between the two members of a dipole) must always be small as compared to the effective wave length. This implies that at high frequencies, a way must be found to "amplify".lambda., i.e., to convert .lambda. into a larger wave length .lambda.' so that L/.lambda.' will be less than 1/2, or in other words, the signal frequency would have to be converted to F', a frequency smaller than the signal frequency. Likewise, for low frequency work, it would be desirable to convert the signal frequency to a higher frequency, thus making the ratio of L, the spatial separation, and the new wave length to be of the order or 1/2 or less. However, the signal frequency cannot readily be converted to a higher or lower frequency while the acoustic wave is in the acoustic medium; and once the signal has been received simultaneously by two hydrophones, the interference patterns preordained by the geometry and the wave length are basically unchangeable. Thus, in order to change the wave length of the acoustic signal at high or low frequencies, it is desirable to devise a system where each received signal, at input frequency f.sub.i, would be apparently heterodyned to produce an arbitrary fixed output frequency f' which is referred to as the post-processing frequency. The fixed spatial separation L will be chosen to be such that it is of the order of .lambda.'/2 where .lambda.' is the wave length associated with frequency F' . The frequency response of a dipole pair then would no longer be limited at the upper end of the band by the dipole peaking frequency but rather by the first mechanical resonance of the individual monopole.